This application was filed under 35 U.S.C. 371, and is the national stage of PCT/EP00/00441, filed Jan. 21, 2000.
The present invention relates to a process and a device for the purification of crystallizable compounds by means of optionally fractional crystallization, wherein a molten material or a mixture containing essentially one desired compound and at least one impurity is subjected to at least partial crystallization.
At present, chemical starting materials and intermediates, particularly starting materials used in polymer production, are produced in enormous quantities. In order to meet the increasing demands with respect to quality, impurities in these products must be virtually absent.
In addition to distillation, crystallization has increasingly been used as a method of purification in recent years. The latter method is advantageous in that impurities which cannot be removed by distillation may frequently be removed by means of crystallization.
However, crystallization involves the drawback that impurities possibly present in the molten material will also precipitate during crystallization. It is a well-known fact that the desired product is depleted in the mother liquor or molten material during crystallization, and that impurities accumulate accordingly. Thus, some of the impurities reach and exceed their solubility limit in the course of the purification process. The impurities will precipitate as soon as the solubility limit is reached. These undesirable crystals then undergo sedimentation on the bottom or the walls of a crystallizer. As these crystals adhere to the surfaces, they will remain in the crystallizer when discharging the non-crystallized molten material. Then, when melting the deposited layer of crystals, where normally two or more fractions are formed, the precipitated impurities still present on the bottom and the walls of the crystallizer will be reabsorbed by the heated molten material. As a consequence, the impurities can never be removed completely and concentrate in the crystallizer.
If the desired compound is purified at least partially in a static crystallizer, the impurities will also be incorporated in the crystallized material because there is no agitation in the molten material.
For example, problems of the above-mentioned type occur in the purification of acrylic acid. Depending on the production process and the efficiency of the subsequent distillation, the prepurified acrylic acid will contain a more or less large proportion of maleic acid and phenothiazine (PTZ) which are by-products in the production of acrylic acid. According to a purification process suggested in EP-A-0,616,998, acrylic acid is purified using a combination of dynamic and static crystallization, wherein the residue from the dynamic crystallization is purified using static crystallization, and the purified acrylic acid is refed into the dynamic crystallization. As a result, the molten acrylic acid material which is refed into the static crystallizer already contains a high percentage of by-products such as maleic acid and phenothiazine (PTZ). When cooling the molten material, maleic acid and phenothiazine (PTZ) are precipitated and part thereof will subsequently be incorporated in the deposited layer of crystals.
Thus, in an industrial purification plant, several hundred kilograms of maleic acid may be formed during one day. Obviously, the solid maleic acid may block pipes and valves. Also, this maleic acid may precipitate as a solid deposit on the crystallizer walls or the bottom thereof. Therefore, it was found necessary to remove these precipitates.
Various solutions have been suggested to separate the precipitated by-products from the molten material. One solution suggests removal of the precipitated impurities by filtration. For this purpose, a static crystallizer 11 is connected with a separator 13 as illustrated in FIG. 1. Normally, the separator 13 can be a filter, but also, a centrifuging device, a suction filter, or any other device allowing separation of solid and liquid materials. The separator 13 is connected with the crystallizer 11 via pipes 15, 17. The pipe 17 is connected to tubes 19, 21 arranged on opposite sides in the bottom area of crystallizer 11. The tubes 19, 21 have a number of openings 23 preferably oriented at an angle to the crystallizer bottom. A collector channel 25 is provided in the center of the bottom 24 which is designed in a V-shaped cross-section. The collector channel is connected with separator 13 via pipe 15. Pipe 15 is provided with a pump 27 which is used to circulate the molten material.
The removal of impurities precipitating during the crystallization process is effected in such a way that molten material is withdrawn through pipe 15 by means of pump 27 and conveyed through separator 13 wherein the solids are retained. The molten material discharging from separator 13 is then refed into crystallizer 11 via pipe 17. Preferably, the flow rate of the molten material discharging from the openings 23 is selected in such a way that a laminar flow will form within the area of crystallizer bottom 24. In this way, the circulation of molten material will not interfere with the crystallization process. By continuously withdrawing the bottom layers of the molten material, it is possible to remove the gradually depositing impurities. Obviously, such a separator can be combined with any type of crystallizer where the product is deposited on cooled surfaces (e.g., falling film or static crystallizers). However, incorporation of precipitating impurities in the crystals cannot be completely avoided when using the separator as described. Also, it was found that not only maleic acid but also other compounds may undergo precipitation.
It is therefore the object of the present invention to provide a process and a device by means of which the problems mentioned above can largely be avoided. In particular, said process and device should allow for efficient purification of products where purification using fractional crystallization is impeded or made impossible due to precipitating impurities, and for further reduction of the percentage of residual impurities. Another object is to provide an improved process and a device for the purification of acrylic acid.
According to the invention, this is done by using a process wherein in case at least one impurity is present, which undergoes precipitation during the purification process as a result of surpassing the solubility limit, the molten material or the mixture to be purified is added with a solvent or a mixture of solvents in such an amount that the respective impurity is retained in solution.
The solvent or the mixture of solvents preferably has a high affinity to at least one of the impurities, i.e., the solvent preferably has a superior dissolving capacity for the impurity compared to the product to be purified. This is advantageous in that only a small amount of solvent has to be added to the molten material in order to prevent precipitation of the impurities. Moreover, only a small amount of product to be purified will dissolve in the solvent in this case, so that the efficiency of the process is maintained.
Conveniently, the suitable solvent or mixture of solvents is selected according to the well-known solubilities of these substances in the respective solvents. As is well-known, polar compounds have particularly good solubility in polar solvents and apolar substances in apolar solvents, for example. In order to increase the solubility of specific impurities, it is particularly preferred to add a solvent to the molten material, which is already present as an impurity in the employed material.
The inventive advancement of the well-known crystallization processes may be used in the purification of any material during which precipitation of impurities occurs. Fractional crystallization is preferably used as crystallization method. In particular, materials to be purified using fractional crystallization are acrylic acid, methacrylic acid, waste water, methylenediphenylisocyanate (MDI), toluenedi-isocyanate (TDI), caprolactam, benzoic acid, bisphenol A, nitrochlorobenzene, straight-chain and branched fatty acids, hydrazine, phenols such as para-, meta- and ortho-cresol, 2,6- and 3,5-dimethylphenol, naphthol and o,o-diophenol, chlorinated hydrocarbons such as dichlorobenzene and nitrochlorobenzene, naphthalene, 1-,2-methylnaphthalene, acenaphthene, fluorene, phenanthrene, adipic dinitrile, hexamethylenediamine, as well as paraffins from C17 on.
In particular, the process according to the invention is suitable in the purification of acrylic acid by removing maleic acid and/or its anhydride and other impurities, if necessary.
The present invention is also directed to a crystallization device and plant which differs from well-known devices in that the device is provided with a connecting piece for metering a solvent or a mixture of solvents. The solvent preferably is added to a holding tank or a tank for intermediate storage of the fractions obtained. It is also preferred to add the solvent directly to the crystallizer.